Add a dashboard to Leafpub

[karsasmus]

This issue is for planning the dashboard

The dashboard can be your entry point after logging in to Leafpub instead of posts view.
It's an optional feature. You can enable/disable it via the setting's advanced tab.

It is customizable with widgets.
A widget is a Leafpub plugin that adds an element to the dashboard.

Widgets can be added via locater (cmd + p).
Widget's size and position can be changed with drag'n drop and is saved automatically to the database.

Tasks

• Add a model to interact with widgets
• Add API endpoints to save the dashboard's layout, get a rendered widget
• implement widgets
• change redirect in AdminController::dashboard